1. Field of the Invention
The present invention relates to a dielectric filter provided with a dielectric block and a conductive cover, a dielectric duplexer, and a communication apparatus including the dielectric filter and the dielectric duplexer.
2. Description of the Related Art
A dielectric filter which has a substantially-rectangular-parallelepiped dielectric block having resonator holes arranged in parallel and internal conductors formed on internal surfaces of the resonator holes, and which has external conductors formed on the filter has been used as a microwave filter.
In the dielectric filter of the above type, in order to eliminate a problem of unnecessary radiation of electromagnetic waves from an open circuit surface of the dielectric filter to the exterior, or unnecessary coupling with the exterior, a conductive cover is provided to cover the open circuit surface of the dielectric block (e.g., Japanese Unexamined Patent Application Publication No. 9-167902).
The conductive cover (of the related art) of the dielectric filter of the above type is formed by simply bending a metal plate at a substantially right angle in one position.
Accordingly, there is a possibility that the conductive cover may be deformed by some external force while the dielectric filter is produced or when the finished dielectric filter is mounted on a circuit board in an electronic device, or after the mounting step. If the conductive cover is shaped to have an acute bending angle, the distance between an open circuit surface of the dielectric block and the conductive cover at a position where they face each other changes, thus changing the electrical characteristics of the filter. Also, if the conductive cover has an obtuse bending angle, the distance between the open circuit surface of the dielectric block and the conductive cover in a position where they face each other changes, so that, in general, the electromagnetic shielding effect decreases.
In the dielectric filter provided with the conductive cover of the related art, coupling electrodes provided on the open circuit surface of the dielectric filter are used to establish coupling between resonators, or coupling between a predetermined resonator and the exterior.
In this case, since two adjacent resonators are easily coupled, the distance between two resonators not to be coupled must be increased. This is part of the reason why reduction in the size of the dielectric filter size is prevented.